The invention is a process for separating C8 alkylaromatic hydrocarbons from C9 and C10 alkylaromatic hydrocarbons.
C8 alkylaromatic hydrocarbons are generally considered to be valuable products, and para-xylene in particular is in high demand. On the other hand, C9 and C10 alkylaromatic hydrocarbons are not nearly as valuable but are typically produced as a byproduct in the same aromatic production processes used to produce C8 alkylaromatic hydrocarbons. Various approaches have been used to convert the less valuable C9 and C10 alkylaromatic hydrocarbons into C8 alkylaromatic hydrocarbons. One popular approach has been to transalkylate C9 and C10 alkylaromatic hydrocarbons along with benzene or toluene to form the C8 alkylaromatic hydrocarbons. Specifically, trimethylbenzenes and tetramethylbenzenes have been transalkylated along with benzene and toluene to form xylenes. However, transalkylation reactions are equilibrium limited and the product contains a mixture of unreacted C9 and C10 alkylaromatic hydrocarbons along with the desired C8 alkylaromatic hydrocarbons. To increase conversion, commercial processes have utilized a two-stage design with the first stage being a fixed bed reactor and the second stage being a separation unit. Unreacted C9 and C10 alkylaromatic hydrocarbons present in the reactor product stream are separated and recycled to the reactor; see for example U.S. Pat. No. 3,211,798 B1.
Once the C8 alkylaromatic hydrocarbons have been produced, they may need to be separated from the unreacted C9 and C10 alkylaromatic hydrocarbons. The present invention provides a process for separating the desired C8 alkylaromatic hydrocarbons from the less desired C9 and C10 alkylaromatic hydrocarbons using zeolite Y, or ion exchanged zeolite Y as an adsorbent. Zeolite Y has been used as an adsorbent in other applications such as the separation of the specific C8 alkylaromatic hydrocarbon isomers. For example, U.S. Pat. No. 4,255,607 B1 discloses the separation of aromatic C8 isomers by adsorption, preferably contacting the mixture with zeolite Y and then developing the resulting adsorption band with an ether having selectivity for para-xylene. Japanese Patent No. 79,037,129-B discloses contacting a mixture of C8 aromatic hydrocarbons with a Y-type zeolite containing sodium, calcium, cobalt and or strontium as cation to selectively adsorb meta-xylene. U.S. Pat. No. 4,079,094 B1 discloses separating ethylbenzene from a mixture of xylene isomers by passing through a column of an adsorbent comprising type X or Y zeolite completely exchanged with strontium and potassium. The xylenes are selectively adsorbed and an ethylbenzene stream is withdrawn. U.S. Pat. No. 4,028,428 B1 discloses separating ethylbenzene from a mixture of xylene isomers by contacting the mixture with an adsorbent of a strontium-exchanged type X or type Y zeolite. The xylenes are selectively adsorbed and ethylbenzene may be withdrawn. U.S. Pat. No. 3,998,901 B1 discloses separating ethylbenzene from a mixture of xylene isomers under adsorption conditions with a type X or Y zeolite completely exchanged with strontium and potassium. U.S. Pat. No. 3,997,620 B1 discloses para-xylene being separated from mixtures containing other C8 aromatics by contacting the mixture under adsorption conditions with type X or Y zeolite containing barium and strontium which selectively adsorbs the paraxylene.
The present invention solves a different problem from that of separating C8 alkylaromatic hydrocarbon isomers. Instead, the present invention is directed to separating at least one C8 alkylaromatic hydrocarbon from at least one C9 or C10 alkylaromatic hydrocarbon, which is a problem encountered in processes such as transalkylation. U.S. Pat. No. 4,956,552 B1 teaches that p-ethyltoluene may be separated from a mixture comprising p-ethyltoluene and at least one other component selected from C8 alkylaromatic hydrocarbons and other C9 aromatic hydrocarbons by contacting the mixture with zeolite Y ion exchanged with potassium. The p-ethyltoluene is selectively adsorbed and a raffinate stream containing the less strongly adsorbed alkylaromatic hydrocarbons is produced. The zeolite Y ion exchanged with potassium is contacted with a desorbent comprising 1,2,3,4-tetrahydronaphthalene or lower alkyl derivative thereof or an alkyl derivative of naphthalene at desorption conditions to effect the removal of p-ethyltoluene from the adsorbent as an extract stream. Applicants have discovered that in addition to zeolite Y ion exchanged with potassium, sodium zeolite Y as synthesized, without ion exchange, is effective to separate at least one C8 alkylaromatic hydrocarbon from at least one C9 or C10 alkylaromatic hydrocarbon having at least one methyl or ethyl group, or a mixture thereof. Similarly, applicants have found that dealuminated sodium zeolite Y is successful in the claimed separation.
The purpose of the invention is to separate at least one C8 alkylaromatic hydrocarbon from at least one C9 or C10 alkylaromatic hydrocarbon. The invention involves contacting a mixture containing (I) at least one C8 alkylaromatic hydrocarbon and (II) at least one C9 or C10 alkylaromatic hydrocarbon having at least one methyl or ethyl group, or a mixture thereof, with dealuminated zeolite Y having a SiO2/Al2O3 ratio in the range of from about 7 to about 25 and containing essentially sodium in the ion-exchangeable sites to selectively adsorb the C9 or C10 alkylaromatic hydrocarbon. The C9 and/or C10 alkylaromatic hydrocarbon(s) are more strongly adsorbed by the adsorbent relative to the C8 alkylaromatic hydrocarbon. The C8 alkylaromatic hydrocarbon may pass through the adsorbent or if weakly adsorbed, may be desorbed using a desorbent and is collected. The more strongly adsorbed C9, C10, or mixture of C9 and C10 alkylaromatic hydrocarbon(s) is desorbed using the desorbent and collected. In a more specific embodiment of the invention, the desorbent is selected from toluene, benzene, or a mixture thereof. In another more specific embodiment of the invention, the adsorbent is dealuminated zeolite Y having a SiO2/Al2O3 ratio in the range of from about 7 to about 12.